1. Field of the Invention
The present invention generally relates to downhole tools for use in a wellbore. More particularly, the invention relates to a downhole tool for sealing a wellbore, such as a hydrocarbon wellbore. More particularly still, the invention relates to an inflatable sealing element for a downhole tool used for sealing a hydrocarbon wellbore.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed, and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
After a well has been drilled and completed, it is desirable to provide a flow path for hydrocarbons from the surrounding formation into the newly formed wellbore. To accomplish this, perforations are shot through the liner string at a depth which equates to the anticipated depth of hydrocarbons. Alternatively, a liner having pre-formed slots may be run into the hole as casing. Alternatively still, a lower portion of the wellbore may remain uncased so that the formation and fluids residing therein remain exposed to the wellbore.
When a wellbore is completed, the wellbore is opened for production. In some instances, a string of production tubing is run into the wellbore to facilitate the flow of hydrocarbons to the surface. In this instance, it is common to deploy one or more packers within the tubing string in order to seal the annular region defined between the tubing and the surrounding string of casing. In this way, a producing zone within the wellbore is isolated.
Various types of packers may be utilized. One common type of packer is an inflatable packer. Inflatable packers employ an elongated bladder that is inflated using a working fluid or well fluids. Inflation may be accomplished either by injecting fluid into the borehole from the surface, or through actuation of a downhole pump.
Inflatable packers are commonly used to seal the annular space around a string of production tubing in order to direct the flow of production fluids up the bore of the tubing and to the surface. However, inflatable packers may be used for many other purposes during the life of a well. For example, an inflatable well packer may be used to seal the annulus between a liner string and a surrounding string of casing during well completion. They may be used to support a column of cement above a lost circulation zone. They may also be used to isolate producing zones from cement contact during a cement squeeze job.
An inflatable packer may also be used to affect a complete seal of a tubular bore at a selected depth in a wellbore. In this instance, the inflatable packer is more commonly known as a bridge plug. In some instances, a bridge plug may be used to permanently plug a well after production operations have ceased. In other instances, a wellbore may be temporarily plugged so that formation treatment operations may be conducted. For example, a bridge plug may be set at a depth below a production zone within the casing. A formation treating operation can then be conducted above the bridge plug by injecting gel and sand, under pressure, into the formation. Still other uses for packers are also known, including dual use as an anchor.
For purposes of this disclosure, the term xe2x80x9cbridge plugxe2x80x9d will be used to refer to and to include any downhole tool which includes an expandable bladder as part of a sealing element, or xe2x80x9cpacking element.xe2x80x9d This includes devices having a throughbore that would more commonly be considered xe2x80x9cpackers.xe2x80x9d
The bladder in a typical inflatable bridge plug is surrounded by two separate expandable cover portions. The first cover portion is an expandable anchor; the second cover portion is an expandable sealing cover. Together, the bladder and the two surrounding cover portions make up a xe2x80x9cpacking element.xe2x80x9d
First, the expandable anchor portion of a packing element serves to frictionally engage the surrounding case or, as the case may be, the raw borehole. Typically, the anchor portion defines a series of vertically overlaid reinforcing straps that are exposed to the surrounding casing. The straps are aligned along the linear plane of the tool so as to essentially run the length of the packing element. At the same time, the straps are placed radially around the bladder in a tightly overlapping fashion. For this reason, the straps are sometimes referred to as xe2x80x9clapped steel ribsxe2x80x9d. The ends of the metal straps are welded together and are secured to end collars. One end collar defines a slidable sub which permits that end to be drawn up as the reinforcing straps are expanded. Upon expansion, the straps engage the surrounding pipe, serving to anchor the bridge plug within the wellbore. Sufficient straps are employed so that as the bladder expands the straps, the straps do not completely separate, but retain the bladder therein.
As an alternative to the use of metal straps, woven or braided steel cable may be used. In the case of a braided cable reinforcement, a closed tube of braided material is secured at opposite ends to packer end collars. A compression assembly is provided between a pair of conical clamping surfaces for securing the cables. In some cases, the end attachment of braided reinforcement is supplemented by injection of an epoxy polymer between the interstices of cable and the conical clamping surfaces.
As noted, the second cover portion of the inflatable bridge plug is the expandable sealing cover. The sealing cover defines a pliable material which surrounds a portion of the reinforcing straps (or other anchor portion). As the bladder and straps are expanded, the sealing cover expands and engages the surrounding pipe in order to effectuate a fluid seal. Thus, the anchor portion and the sealing cover portion of the packing element combine to effectuate a setting and sealing function for the bridge plug.
Inflatable bridge plugs enjoy certain advantages over mechanically set bridge plugs/packers. Primarily, inflatable bridge plugs are advantageous in the context of high expansion operations. In this respect, most inflatable bridge plugs are capable of achieving a higher expansion ratio than mechanically set bridge plugs and packers. Those of ordinary skill in the art will understand that the expansion ratio is defined by the ratio of the inside diameter of the surrounding pipe to the original outside diameter, i.e., running diameter, of the packing element. However, high expansion applications (typically those greater than 2.25:1) place challenges on the designer to balance the anchoring and sealing capabilities of the packing element. In this regard, a trade-off oftentimes occurs in the design of a bridge plug between a high sealing capability and a high anchoring capability. A higher expansion ratio typically affords a greater anchoring capacity for the straps; in contrast, a lower expansion ratio provides for a weaker anchoring contact between the straps and the surrounding pipe.
In an effort to accomplish both a strong anchoring function and a strong sealing function for an inflatable bridge plug, designers have offered various configurations for the packing element. For example, in one arrangement an elongated sealing cover is provided, with the sealing cover being open or xe2x80x9cexposedxe2x80x9d central to the anchor. In this arrangement, the anchor portion is located in the center of the packing element. However, because the anchor portion is short relative to the sealing cover portion, this arrangement compromises the maximum anchoring capability of the bridge plug. In this respect, due to the shape change that occurs in the element under load, the short anchor in the center of the element will not distribute the applied differential load through the anchor to the pipe wall as efficiently as an anchor placed toward the end of the packing element. The shape change can occur because the inner mandrel within the bladder and the control valve tends to xe2x80x9cfloatxe2x80x9d along the central line of the packing element, allowing the bottom of the packing element to slide along the mandrel. Contact to the pipe wall is made via the reinforcing metal strap and rubber cover. As load is applied to the packing element from below, the element can bunch up. In contrast, as load is applied from above, the element tends to morph from a circular cylinder shape to a teardrop shape. Hence, the metal reinforcing straps do not uniformly bite into the surrounding pipe. However, this arrangement does provide an optimum seal with the surrounding pipe wall due to the long rubber cover on either side of the anchor.
In an effort to overcome the problem of the short center anchor, some have offered a long anchor located in the center of the packing element. Typically, a long anchor would be a length in excess of 20 inches. This longer anchor will provide a stronger grip with the surrounding pipe. However, the sealing efficiency is reduced due to the shorter cover lengths on either side of the exposed reinforcing straps.
Another arrangement for the packing element which has been designed offers two long anchors on opposite ends of the packing element, with a short sealing cover in the middle. This arrangement provides an acceptable bi-directional anchor for reinforcing the surrounding pipe. However, this dual anchor design tends to capture fluid between the two anchoring ends as they expand, preventing full expansion of the intermediate sealing cover. The short cover is sometimes an ineffective seal as it allows fluid to bypass between the reinforcing straps and the underside of the cover. In addition, strap buckling can occur within the reinforcing straps as they expand, causing a catastrophic failure of the bridge plug.
To overcome this problem, packing elements have been offered utilizing only a single anchor portion and a single sealing cover portion. In one known arrangement, a short anchor is placed at one end of the packing element, and a longer sealing cover is maintained at the opposite end of the packing element. However, a short anchor biased to one end of the packing element will not grip the surrounding pipe sufficiently to prevent sliding of the bridge plug at the maximum designed differential pressure unless higher initial inflation pressures are used. Further, a short anchor is less effective in low expansion applications.
As can be seen, an improved packing element for an inflatable bridge plug is needed. More specifically, a packing element is needed which employs a longer anchoring portion which is biased at one end of the bladder. Further, a need exists for an inflatable packing element which maximizes both the anchoring and sealing functions of an inflatable bridge plug.
The present invention provides an inflatable packing element for use on a bridge plug. In the packing element of the present invention, an expandable anchoring portion is placed at one end of the packing element, while a pliable, expandable sealing cover portion is placed at the opposite end of the packing element. The length of the anchor portion is longer than in known inflatable bridge plugs wherein the anchor is biased to one end. The increased anchor length serves to insure that the inflatable bridge plug will not slide after being set within casing at low expansion ratios, as well as at higher expansion ratios (up to and in excess of 3:1).
The length of the anchor is determined by a novel calculation which considers the coefficient of friction between the reinforcing straps of the anchoring portion and the surrounding pipe wall. The calculation also considers the area of pipe contact as well as contact pressure generated from the bladder of the bridge plug. The length of the anchor portion upon expansion is at least approximately 2.63xc3x97 the inner diameter of the opening of the wellbore, e.g., surrounding casing. At the same time, the length of the anchor portion is no greater upon expansion than approximately 49% of the total length of the expanded packing element, that is, the length of the anchor portion engaging the surrounding wellbore opening plus the length of the sealing cover portion engaging the surrounding wellbore opening.
It is desired, though not required, that a pliable cover ring be placed around the welded metal straps of the anchor portion at one end, and the sealing cover portion be circumferentially disposed around the anchor portion at an opposite end.